Four things that have changed about enclosed space entry
IMO Resolution MSC.581(110) took effect in December 2025 and introduces revised recommendations for enclosed space entry aboard ships, including new requirements for gas detection and CO₂ monitoring. While it is an IMO recommendation, flag administrations and port state control inspectors are increasingly applying it as a benchmark. This means a vessel may comply with the letter of SOLAS but still face findings during an inspection if it does not meet the revised recommendations.
Here are four important changes to understand and what they mean for your onboard procedures and equipment.
What has changed?
CO2 is now considered a primary hazard
Carbon dioxide is explicitly classified as a primary atmospheric hazard for the first time. It is odourless, gives no warning, and causes rapid unconsciousness. Pre-entry testing should now include CO2 measurement.
Connected spaces and trapped atmospheres are in scope
The definition of hazardous areas now includes any space linked to a source space, and addresses pockets of gas that persist after ventilation.
Solo entry is prohibited
At least two people must be present for any enclosed space entry to ensure immediate rescue capability.
Entry is only permitted when stable readings show
| Oxygen (O2) | +/- 20.9 vol% |
| Carbon dioxide (CO2) | Below 0.5% (5,000 ppm) |
| Flammable gases | Below 1% LEL |
| Toxic gases | Below 50% of occupational exposure limit (OEL) |
What compliance requires
Competent person - Atmospheric testing and risk assessments must be conducted by a “competent person” with documented theoretical and practical training.
Permit and continuous monitoring - A written permit and continuous personal air monitoring are required for every entry.
Detection equipment - All ships must carry at least two sets of gas detection equipment. Vessels carrying high-risk cargoes such as coal, wood pellets or metal sulphide concentrates must carry at least four.
Where CO2 risk comes from on your vessel
CO2 hazards are not the same on every ship. The source depends on what your vessel carries and how it operates. These are the most common sources to assess:
Vessel type |
Primary CO2 source |
Focus areas |
| Bulk carriers | Fermentation of organic cargo such as grain, wood chips or fish meal naturally absorbs oxygen and releases CO2 during transit. Coal oxidation adds to the risk. | Cargo holds, access trunks |
| Tanker | Inert gas systems using CO2-rich flue gas to inert tanks. Leaks in pipe tunnels or adjacent spaces can build up without warning. | Pump rooms, double hulls, void spaces |
| Container and cargo vessels | Leaking or accidentally activated fixed CO2 fire suppression systems (All vessel types). Refrigerated containers using CO2 as coolant. | Hold spaces, CO2 rooms, ventilation ducts |
| Ro-Ro and ferries | Exhaust gas from vehicles, main engines or generators migrating into poorly ventilated spaces or drawn in through ventilation intakes. | Car decks, ramp machinery spaces |
| CO2 carriers | Vessels transporting captured CO2 face high-pressure leak risks that can overwhelm a compartment rapidly. | Cargo containment areas, adjacent machinery spaces. |
| Vessels with onboard refrigeration | Systems using CO2 as a refrigerant or cooling organic matter can experience leaks into enclosed spaces. | Refrigeration machinery spaces, adjacent areas. |
Our recommendation for different vessel types
X-am 5000/5600/8000 equipped as standard with 5 gases: Ex / O2 / CO / H2S / CO2
This advisory summarises IMO Resolution MSC.581(110) and its application through inspection and audit. It is general guidance, not legal advice.